I have 'emerged' digikam 2.6.0, and everything seems to work correctly, but the metadata options 'Read from sidecar files' and 'Write to sidecar files' are grayed out, so I can't activate them. Also, when activating the metadata information of an image (through the 'Metadata' button on the right side bar), three tabs appear ('EXIF', 'IPTC' and 'Maker notes'), but the tab for XMP information is missing.
I have also installed the kipi-plugins.

Thanks amair. I checked that I have the xmp USE flag, and reinstalled exiv2 with no luck.

Finally, the Digikam team gave me the solution: the problem is not with exiv2, but libkexiv2. I reinstalled those libraries and that solved the problem(maybe these libraries got installed from some dependency before I added this flag... )